1. Field of the Invention
The present invention relates to a lithium-ion secondary battery.
2. Description of the Related Art
In the related art, laminar compounds such as LiCoO2 and LiNi1/3Mn1/3Co1/3O2 and spinel compounds such as LiMn2O4 have been used as a positive electrode material (positive electrode active material) of a lithium-ion secondary battery. In recent years, olivine-type compounds represented by LiFePO4 have attracted attention. The positive electrode material having the olivine structure has been known to have high thermal stability at a high temperature and to have a high safety.
However, a lithium-ion secondary battery using LiFePO4 has defects that a charging/discharging voltage thereof is about 3.5 V, which is low, and energy density is lowered. Therefore, LiCoPO4, LiNiPO4 and the like have been proposed as phosphate positive electrode materials capable of realizing a high charging/discharging voltage. However, at present lithium-ion secondary batteries using these positive electrode materials cannot obtain a sufficient capacity. As a compound capable of realizing a 4V-grade charging/discharging voltage among the phosphate positive electrode materials, vanadium phosphates having structures of LiVOPO4 and Li3V2(PO4)3 has been known.
It is not known that gas generation occurs in the lithium-ion secondary battery using the vanadium phosphate compound in some cases. However, gas is actually generated and the lithium-ion secondary battery is swollen to cause a problem that shape stability is impaired. In particular, when a metal laminated case is used, a change in the shape thereof is remarkable.
While there is a disclosure about vanadium phosphate in Japanese Unexamined Patent Application No. 2004-303527, a problem of gas generation is not disclosed. In Japanese Unexamined Patent Application No. 5-6778, in order to suppress gas generation, the addition of a small amount of water soluble polymer to a conductive carbon paste solution in a conductive layer on a negative electrode body is disclosed. However, a case in which vanadium phosphate is used as a positive electrode active material is not disclosed nor suggested.